Nihonmatsu Yoshitsugu served as a bushō during the Sengoku and Azuchi-Momoyama periods. He was the ninth head of the Nihonmatsu clan and served as the lord of Nihonmatsu Castle in the Adachi District of Mutsu Province.
In 1552, Yoshitsugu was born as the eldest son and designated heir of Nihonmatsu Yoshikuni, the eighth head of the Nihonmatsu clan.
In 1580, Yoshitsugu is surmised to have succeeded his father, Yoshikuni, as the head of the clan preceding the death of Yoshikuni later that same year. Ōba Sanzaemon served as a retainer of Yoshitsugu for a while before being transferred to serve Ashina Moritaka.
In 1585, Date Masamune (the seventeenth head of the Date clan and a sengoku daimyō of Mutsu and Dewa provinces), together with his father-in-law, Tamura Kiyoaki, attacked Ōuchi Sadatsuna. Yoshitsugu was connected to Sadatsuna through family marriage, so was also attacked by Masamune. Yoshitsugu offered to surrender to Masamune, but Masamune refused the offer and, with the exception of a small parcel near Nihonmatsu, all of Yoshitsugu’s landholdings were seized by the Date. This caused Yoshitsugu to lose his status as a daimyō in Mutsu. Through the offices of Masamune’s retired father, Date Terumune, and Date Shigezane, these conditions were alleviated but Masamune deeply resented Yoshitsugu.
On 10/8 of Tenshō 13 (1585), when Yoshitsugu paid a visit to Miyamori Castle within the territory of Terumune, he abducted Terumune and attempted to take him back to Nihonmatsu Castle. While en route on the Takada Plain, he was chased down by Masamune and either died when he and Terumune stabbed one another or was shot and killed along with Terumune and the others in the party. Yoshitsugu was thirty-four years old. His remains were desecrated by Masamune, tied together with a wisteria vine, and ruthlessly suspended in a tree.